Project Summary/Abstract The p53 tumor suppressor plays a critical role against tumorigenesis by regulating numerous growth inhibitory pathways in response to a plethora of biological stresses. Several reports have implicated single nucleotide polymorphisms (SNPs) causing impaired p53 function and tumor development. One such SNP affects codon 47 of TP53 and encodes a serine (S47) instead of a proline. Approximately 5-8% of Africans and 2.5% of African-Americans express a p53 allele with S47. How this population-specific variant might impair p53 function, in turn influencing genetic susceptibility to disease and response to therapy, remains unclear. We have recently generated a mouse model to gain a better understanding of the role of the S47 variant in disease. This model utilizes a humanized p53 knock-in (Hupki) model in order to recapitulate the activity of human p53 in mice. We also generated WT and S47 transformed cells to determine their response to therapy. While S47 transformed MEFs showed reduced sensitivity to most chemotherapeutic agents, we found that they are 5-fold more sensitive to a class of epigenetic drugs known as BET inhibitors. Furthermore, S47 mice have a higher incidence of spontaneous cancer, with over 30% of these mice developing hepatocellular carcinoma (HCC) by 12-18 months of age. We found that the S47 variant has impaired ability to transactivate a subset of p53 target genes involved in ferroptosis, an iron-mediated, caspase independent form of cell death. Thus, it seems plausible that S47 mice are more prone to HCC in part because of dysregulated ferroptosis. The long-term goal of this study is to identify therapeutic strategies to target S47 tumors, and to elucidate the transcriptional and functional defects of the S47 variant. We propose the following specific aims: Aim 1: To test the hypothesis that BET inhibitors preferentially target S47 tumors compared to WT. Aim 2: To define the contribution of ?S47-impaired? genes to ferroptosis and suppression of liver cancer. We propose to test two BET inhibitors currently in clinical trials using a xenograft model of S47 cancer. We also recently completed a comprehensive analysis of differentially expressed, S47-impaired target genes, by performing RNA-seq in our mouse model, and identified 6 differentially expressed genes that are known p53 targets and/or play a role in ferroptosis. We will validate the role of these genes in ferroptosis, iron accumulation, and transformation. We expect this study to identify critical, p53-mediated, biomarkers of ferroptosis, and to elucidate a genetic basis for increased liver cancer incidence and response to epigenetic therapy in African-Americans possessing the S47 variant.